MOS high frequency switch circuit using a variable well bias

ABSTRACT

An RF switch comprises a switching FET having gate and back gate terminals, an input port for receiving an RF signal, and an output port for providing substantially the RF signal during an ON state of the FET. Switching circuitry connects the back gate terminal of the FET to the input port during the ON state to reduce insertion loss during the ON state, and connects the back gate terminal to a point of reference potential during an OFF state of the FET to increase isolation during the OFF state. Preferably, the switching FET is a depletion mode silicon MOSFET capable of operating with low supply voltages. The switching circuitry preferably comprises a second FET for electrically connecting the back gate terminal and the input terminal (e.g., source) of the switching FET during the ON state, and a third FET for electrically connecting the back gate terminal of the switching FET to the point of reference potential during the OFF state.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to metal-oxide-semiconductor (MOS) devices fabricated on silicon substrates, and more particularly to MOS devices used as analog high-frequency switches.

2. Description of Prior Art

High frequency field effect transistors (FETs) are often used in integrated radio-frequency (RF) circuits as analog switches to either block a high-frequency signal or feed it through at minimum ac resistance. At the required frequencies in the GHz-range, historically III-V semiconductor transistors, and particularly metal-semiconductor FETs (MESFETs), have been used. Optimization of these devices has been concerned basically with the minimization of the channel "on-resistance". The III-V substrates are semi-insulating and the source/drain (S/D) junction capacitances are very low. As such, the channel width can be increased to reduce the on-resistance without prohibitively raising the S/D capacitance. On the other hand, a disadvantage of III-V switches is that their threshold voltages are typically high, thus requiring a high supply voltage. Also, it is difficult to achieve very high levels of integration in II-V technology, which makes the realization of single-chip transceivers difficult. These two shortcomings make silicon technology, which recently proved to be feasible for GHz circuit design, very attractive for low-cost, low-power (e.g., portable) transceivers.

Silicon metal-oxide-semiconductor (MOS) FETs can be used for switching circuits in the same way as MESFETs, but they have considerable junction capacitances. Consequently, by increasing the channel width of a MOSFET, a proportional reduction of the channel resistance is obtained, but by the same degree an increase in the S/D capacitance results. This has the adverse effect of the AC off-resistance being reduced. The ratio of on- to off-resistance is therefore to first order given by the technology and cannot be optimized by varying the device geometry.

FIG. 1 shows a prior art approach to improve the off-resistance. First n-channel FET (nFET) 11 is the actual switch device. A second nFET 12 is connected (in conjunction with an inverter) to short any AC signal that leaks through nFET 11 to ground when nFET 11 is in the off-state. For the second device 12 a similar trade-off applies as for the first switch device: making the transistor small leads to a less effective connection to ground in off-state, while making the device large leads to a high load capacitance for the first nFET 11 in the ON state.

In contrast to III-V MESFETs, silicon MOSFETs offer an additional possibility to optimize the device performance as a RF switch through the back-gate bias effect. In a silicon MOSFET, a potential difference between the back gate region and the source leads to a reduction of the threshold voltage of the device. In most cases the back gate is connected to ground. Some proposals have considered either switching of the back gate between different constant voltages or adjusting the bias continuously during the switching event. Some of the known circuits for continuous bias adjustment prevent the S/D junction from being reverse biased. Other known circuits either maintain a constant voltage between source and back gate, or switch that voltage to zero under certain conditions.

In U.S. Pat. No. 3,720,848, the source of the switch device is connected to the back gate in on-state through another FET in order to maintain a constant voltage between back gate and source and thus a constant on-resistance. In off-state, however, the back gate potential is not controlled and left floating. In U.S. Pat. No. 3,866,064, additional circuitry is used to maintain a potential difference of one forward diode drop. The on-resistance is therefore constant and independent of the switching signal, but not minimum.

Accordingly, a need exists for a high frequency FET switch having both a low on-resistance and a high off-resistance, and which does not require a high supply voltage to operate.

SUMMARY OF THE INVENTION

The present invention relates to an RF switch which comprises a switching FET having gate and back gate terminals, an input port for receiving an RF signal, and an output port for providing substantially the RF signal during an ON state of the FET. Switching circuitry connects the back gate terminal of the FET to the input port during the ON state to reduce insertion loss during the ON state, and connects the back gate terminal to a point of reference potential during an OFF state of the FET to increase isolation during the OFF state.

Preferably, the switching FET is a depletion mode silicon MOSFET capable of operating with low supply voltages. The switching circuitry preferably comprises a second FET for electrically connecting the back gate terminal and the input terminal of the switching FET during the ON state, and a third FET for electrically connecting the back gate terminal of the switching FET to the point of reference potential during the OFF state. The FETs may be either pFETs or nFETs. However, if nFETs are used, a n well is preferably included within a p substrate, and a p well (back gate region) within the n well, such that the n well can be biased with respect to the p substrate to enable the back gate region to be electrically isolated from the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is had to exemplary embodiments thereof, considered in conjunction with the accompanying drawings in which like reference numerals designate similar or identical elements, wherein:

FIG. 1 schematically illustrates a prior art single-pole, single-throw (SPST) RF switch;

FIG. 2 schematically illustrates an embodiment of a SPST switch of the present invention;

FIG. 3 schematically illustrates another embodiment of a SPST switch;

FIG. 4 is a circuit diagram of a prior art single-pole, double-throw (SPDT) RF switch;

FIG. 5 is a circuit diagram of an embodiment of a SPDT switch of the present invention;

FIG. 6A is a cross-sectional illustration of an nFET which can be used in RF switches of this invention; and

FIG. 6B is a cross-sectional illustration of a pFET which can be used in RF switches of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a circuit diagram of a single-pole, single-throw (SPST) RF switch 20, which is a first embodiment of the present invention. In this embodiment, back gate bias switching is only applied to the series FET, designated as FET₁. The series FET and other FETs of switch 20 will be described for exemplary purposes as being nFETs and are shown schematically as such. However, it is understood that designs employing only pFETs or combinations of pFETs and nFETs could alternatively be implemented. (In the circuit diagrams herein, nFETs are symbolized by means of the arrow of the back gate terminal pointing inwardly. The same circuit diagrams are applicable for pFET designs; however, pFETS are symbolized by the back gate terminal arrow pointing outward. Thus, each nFET in the circuit diagrams herein can be replaced by a pFET, and the circuit diagram would then be altered by flipping the arrow on the back gate terminal of that device. The circuit diagrams are otherwise identical).

SPST switch 20 includes an input port 21 where an RF signal is applied, an output port 22, and a switching port 23 which receives a bias signal controlling an ON and OFF status of the switch. The RF signal appears at output port 22 with low insertion loss in the ON state, and with very high insertion loss in the OFF state. Device FET₁ is connected between ports 21 and 22 and includes gate G₁, source S₁, drain D₁, and back gate B₁ contacts. FET₁, as well as the other FETS in the circuit diagrams herein, are preferably silicon MOSFETS operating in the depletion mode. As such, low supply voltages on the order of 2.5V or less can be utilized to bias the devices. The supply voltage is typically applied to the drain via an RF choke (not shown). The source may also receive a bias voltage via an RF choke. Typically, however, the relatively large RF input signal, e.g., 10 mW-1.0 W, provides the bias to the source with respect to the gate in what is known as "large signal operation" of the device. The frequency of the RF signal is typically in the microwave range or slightly less for wireless communications applications, e.g., 900 MHz to 3 GHz. However, the present invention can be practiced over a much wider frequency range.

Gate G₁ of FET₁ is connected to switch port 23, the source S₁ to the input port 21, and the drain D₁ to the output port 22. The back gate B₁ is coupled to source S₃ and drain D₄ of FET₃ and FET₄, respectively. Drain D₃ of FET₃ is connected to input port 21, while the source S₄ of FET₄ is connected to ground potential. The term "ground" as used herein means a point of reference potential with respect to the RF carrying conductor. This will be explained further below. The respective back gate contacts B₃ and B₄ of the FETs are common with ground. The gate G₃ of FET₃ is coupled to switch port 23, while gate G₄ of FET₄ is connected to output port 19 of inverter 18. The input 17 of the inverter is connected to switch port 23. The output 19 of the inverter drives the gate G₂ of the shunt FET, FET₂, which has its source S₂ and back gate B₂ at ground and its drain D₂ coupled to output port 22.

In the ON-state of the switch, a bias control signal applied to input port 23 is in a first state, e.g., V_(GS) =0V, turning FET₁ and FET₃ ON. Also, FET₂ and FET₄ are each OFF, since inverter 18 provides bias of opposite state to the gates of FET₂ and FET₄. With FET₃ ON, the back gate and source contacts B₁, S₁ of FET₁ are electrically connected together through FET₃. This connection of source and back gate regions minimizes the on-resistance of FET₁. Also, in the ON state, FET₂ is off and thus presents a high shunt impedance which does not result in much additional loss for the switch. In the OFF state of the switch, the bias control signal applied to port 23 is in the opposite state, e.g., V_(GS) =3.6V, FET₁ and FET₃ both switch OFF while FET₂ and FET₄ switch ON. As a result, back gate contact B₁ is connected via FET₄ to ground potential (since source S₄ is connected to ground). This maximizes the off-resistance of the series FET, FET₁. Also, FET₂ is ON, which increases the isolation (insertion loss) of the overall switch in the OFF state, since an RF short is provided reflecting most of the power that leaks through FET₁.

It is noted that field effect transistors are symmetrical devices such that either the source or the drain of any given device could be used as the input port to which RF is applied. In FIG. 2, the source S₁ of the series FET is used as the input port and the drain D₁ is the output. Alternatively, the drain can be used as the input and the source as the output--the circuit diagrams would be otherwise identical. The on-resistance of FET₁ would still be minimized and the off-resistance maximized by virtue of the switching action of FET₃ and FET₄.

FIG. 3 shows an embodiment the same type of SPST switch, but with back gate biasing applied to the shunt FET as well. The back gate of FET₂ is not connected to ground as in FIG. 2, but instead to the source S₅ of an additional device FET₅ and to the drain D₆ of another device FET₆. The source S₆ and the back gate contacts B₅ and B₆ are connected to ground. The drain D₅ of FET₅ is connected to the output port 22. With this configuration, during the ON state of switch 25 (i.e., with FET₁ ON), FET₂ is OFF, FET₅ is OFF since its gate G₅ is coupled to G₂, and FET₆ is ON since its gate G₆ is coupled to inverter input 17. Thus, back gate terminal B₂ is connected through FET₆ to ground, whereby the OFF-resistance of FET₂ is maximized and a high shunt impedance is presented (substantially the same shunt impedance as in switch 20 of FIG. 2). During the OFF state of switch 25, the advantage of this embodiment over switch 20 of FIG. 2 becomes apparent. In the OFF state, FET₆ is OFF, while FET₂ and FET₅ are both ON, so that back gate terminal B₂ is connected through FET₅ to drain terminal D₂, thereby minimizing the on-resistance of FET₂. The on-resistance of FET₂ is thus lower than in the configuration of switch 20. As such, the shunt impedance presented by FET₂ is closer to a perfect short circuit, and the insertion loss of switch 25 is improved (higher) in the OFF state.

The circuit concept described above in reference to FIGS. 2 and 3 can be applied in the same way to other types of RF switches. Referring to FIG. 4, a schematic of a prior art single-pole, double-throw (SPDT) switch is shown in the conventional configuration, i.e. with FETs 11, 12, 31 and 32 all having their back gate contacts at ground. The switch consists of one input port 21, two output ports 22 and 30, and a switch control port 23. The circuit substitutes for two SPST circuits (FIG. 1) which have their input ports connected, and which use a single inverter to provide the necessary gate biases.

With reference now to FIG. 5, a SPDT switch 35 in accordance with the present invention is schematically illustrated. The portion of switch 35 that includes FET₁, FET₂, FET₃, FET₄, inverter 18, input port 21 and output port 22 is essentially the same as in switch 20 of FIG. 2. Hence, back-gate biasing is employed for FET₁ whereby FET₃ acts as the switch to connect contacts B₁ to S₁ during the ON state to reduce the on-resistance of FET₁, FET₄ acts as the switch to connect B₁ to ground during the OFF state to increase the off-resistance of FET₁, FET₂ is OFF during the ON state to have close to negligible impact on insertion loss, and ON during the OFF state to increase isolation. The double throw configuration of switch 35 adds series FET, FET₁₁, which is analogous to FET₁, shunt isolation FET, FET₁₂ which is analogous to FET₂, and FET₁₃, FET₁₄ (analogous to FET₃, FET₄, respectively). In the ON state of FET₁, the RF switch path is from port 21 to port 22; in the OFF state of FET₁, the RF switch path is from port 21 to port 30, since gate G₁₁ of FET₁, is connected to the inverter output 19 and FET₁₁ is ON. With FET₁₁ ON, FET₁₃ is ON and thus acts as a switch connecting the back gate terminal B₁₁ to the source terminal S₁₁, thereby lowering the on-resistance of FET₁₁. With FET₁₁ ON, FET₁₄ and FET₁₂ are OFF. In the ON state of FET₁, FET₁₁ and FET₁₃ are OFF and FET₁₄ is on, thus connecting the back gate terminal B₁₁ to the source terminal S₁₄, which is at ground potential. This maximizes the off-resistance of FET₁₁ such that its presence does not affect the performance of the other side of the switch, and vice versa when FET₁₁ is ON and FET₁ is OFF.

It should be readily apparent that the back gate biasing disclosed herein can be applied to other RF switch applications as well, e.g., single pole, multiple throw switches. Moreover, the shunt FETs in the multiple throw embodiments, e.g., FET₂ and FET₁₂ in FIG. 5, can also have back gate biasing applied to enhance their isolation. For instance, two additional FETs analogous to FET₅, FET₆ can be connected to FET₂ of switch 35 to improve isolation in the first RF switch path (port 21-port 22) and two additional FETS analogous to FET₅, FET₆ can be connected to FET₁₂ of switch 35 to improve isolation in the second switch path from port 21 to port 30.

As discussed above, the FETs employed in the various switch circuit embodiments of FIGS. 2, 3 and 5 can be either nFETs or pFETs, although circuit symbols for nFETs are used in the figures. One advantage of using nFETs is that they exhibit a higher mobility as compared to pFETs. Conventional silicon integration processes, however, do not provide nFETs that ate completely isolated electrically. The back gate contacts of conventional nFETs usually have common potential with the substrate, i.e. the back gate potential of a conventional nFET in a RF switch cannot be individually biased. In accordance with the present invention, the integrated process is modified to provide nFETs having back gate regions that can be individually biased. FIG. 6A shows an embodiment of a nFET, designated as nFET₁, which can be employed within the various switching circuits disclosed herein. nFET₁ is fabricated by first forming an n-well 32, typically of n⁻ doping level, in a p type substrate 31. A p⁺ contact region 39 is formed in the substrate outside the n well 32 and electrically connected to terminal SB of suitable metallization. N⁺ contact region 34 is formed within well 32 and connects to metallization forming terminal NW. A p well 33 is formed within n well 32, and includes a p⁺ contact region 35 connected electrically to back gate terminal B of appropriate metallization. It is noted that terminals NW and SB are not shown in the circuit diagrams in FIGS. 2, 3 and 5; however they are preferably included within each FET when nFETs are employed. Source region 35 and drain region 36 of n⁺ conductivity are formed within back gate region 33 and are biased via respective terminals S, D connected thereto. An insulated gate is then formed of suitable oxide 44 and n⁺ polysilicon layer 42 formed on the oxide.

The purpose of the n well 32 is to electrically isolate the p type back gate well 33 from the substrate 31. Typically, n well 32 is biased at 0.5-1.0V higher than the substrate 31, with the respective biasing brought in through terminals NW and SB. The back gate terminal B is then switched between the potential of the device input port, e.g. source terminal S (in the ON state of the device), to the potential of terminal NW (in the OFF state). Hence, the potential at terminal NW is preferably the point of reference potential (e.g., ground or the most negative potential) employed to achieve a maximum off-resistance for the device.

FIG. 6B shows a cross-section of a pFET, designated as pFET₁, which can be used for the FETs of the switches disclosed herein. The back gate terminal B, connected by a n⁺ contact region 48 to n well 40, is switched between a potential of the input port, e.g., source S, in the ON state to a potential of the p substrate 46 in the OFF state. At low supply voltages, a pFET-based RF switch, connected via the back gate biasing arrangement of the present invention, may perform as well or slightly better than a nFET with a fixed back gate potential.

It will be understood that the embodiments disclosed herein are merely exemplary and that those skilled in the art can make many modifications and variations to the disclosed embodiments without departing from the spirit and scope of the invention. All such modifications and variations are intended to be included within the scope of the invention as defined by the appended claims. 

What is claimed is:
 1. A radio frequency (RF) switch, comprising:a switching field effect transistor (FET) having gate and back gate terminals, an input port for receiving an RF signal, and an output port for providing substantially said RF signal during an ON state of said FET; and switching circuitry for coupling the back gate terminal of said FET to the input port during the ON state to reduce insertion loss during the ON state, and for coupling the back gate terminal to a point of reference potential during an OFF state of said FET to increase isolation during the OFF state; wherein said FET is a nFET including a substrate of p type conductivity; a first well of n type conductivity within said substrate; a second well of p type conductivity within said first well; source and drain regions of n type conductivity within said second well; a gate electrode; a p⁺ region within said second well, said back gate terminal being electrically connected to said p⁺ region; and a n⁺ region within said first n type well and outside said second p type well, said n⁺ region being connected to receive a bias potential to bias the potential of said first well with respect to said substrate.
 2. The switch of claim 1, wherein said first well of n type conductivity has a n⁻ doping level.
 3. The switch of claim 1, wherein said point of reference potential comprises ground potential.
 4. The switch of claim 1, wherein said FET comprises a depletion mode silicon MOSFET.
 5. The switch of claim 1, wherein said switching circuitry comprises a second FET for coupling the back gate terminal of said switching FET to the input port of said switching FET during said ON state and a third FET for coupling the back gate terminal of said switching FET to said point of reference potential during said OFF state.
 6. The switch of claim 1, further comprising a shunt FET coupled to the output port of said switching FET and said point of reference potential, said shunt FET being switched off during said ON state and switched on during said OFF state.
 7. The switch of claim 1, wherein said FET comprises a nFET.
 8. The switch of claim 1, wherein said switch is adapted for use at microwave frequencies.
 9. The switch of claim 1, wherein said input port comprises a source region contact of said FET and said output port comprises a drain region contact of said FET.
 10. A radio frequency (RF) switch, comprising:a switching field effect transistor (FET) having gate and back gate terminals, an input port for receiving an RF signal, and an output port for providing substantially said RF signal during an ON state of said FET; and switching circuitry for coupling the back gate terminal of said FET to the input port during the ON state to reduce insertion loss during the ON state, and for coupling the back gate terminal to a point of reference potential during an OFF state of said FET to increase isolation during the OFF state: wherein said FET is a pFET.
 11. A radio frequency (RF) switch, comprising:a switching field effect transistor (FET) having gate and back gate terminals, an input terminal for receiving an RF signal, and an output terminal for providing said RF signal during an ON state of said FET; second and third FETs, each having input, output, gate and back gate terminals, with the back gate terminals of said second and third FETs being directly coupled to each other and to a point of reference potential, the input terminals of said second and third FETs directly coupled to each other and to the back gate terminal of said switching FET, the output terminal of said second FET directly coupled to the input terminal of said switching FET, and the output terminal of said third FET directly coupled to said point of reference potential; wherein during ON and OFF states of said switch, the gate terminals of said switching FET and said second FET are biased such that said switching and second FETs are each in ON and OFF states, respectively, and the gate terminal of said third FET is biased such that said third FET is in OFF and ON states, respectively, said coupling of said output terminal of said second FET to said input terminal of said switching FET being operational to reduce insertion loss during the ON state, and said coupling of said back gate terminal of said switching FET to said third FET operational to increase insertion loss of said switch during the OFF state.
 12. The switch of claim 11, wherein each of said switching, second and third FETs are n channel silicon MOSFETs operating in the depletion mode.
 13. A radio frequency (RF) switch, comprising:a switching field effect transistor (FET) having gate and back gate terminals, an input terminal for receiving an RF signal, and an output terminal for providing said RF signal during an ON state of said FET; second and third FETs, each having input, output, gate and back gate terminals, with the back gate terminals of said second and third FETs being coupled to each other and to a point of reference potential, the input terminals of said second and third FETs coupled to each other and to the back gate terminal of said switching FET, the output terminal of said second FET coupled to the input terminal of said switching FET, and the output terminal of said third FET coupled to said point of reference potential; wherein during ON and OFF states of said switch, the gate terminals of said switching FET and said second FET are biased such that said switching and second FETs are each in ON and OFF states, respectively, and the gate terminal of said third FET is biased such that said third FET is in OFF and ON states, respectively, said coupling of said output terminal of said second FET to said input terminal of said switching FET being operational to reduce insertion loss during the ON state, and said coupling of said back gate terminal of said switching FET to said third FET operational to increase insertion loss of said switch during the OFF state; further wherein the gate terminals of said switching and second FETs are coupled together and to a switch control terminal, and said switch further including: an inverter having an input coupled to said switch control terminal and an output coupled to said gate terminal of said third FET; and a fourth FET operable to increase isolation of said switch in the OFF state, having a gate terminal coupled to the output of said inverter, an input terminal coupled to said output terminal of said switching FET, and an output terminal coupled to said point of reference potential.
 14. The switch of claim 13, wherein said fourth FET having a back gate terminal coupled to its output terminal.
 15. The switch of claim 13, wherein said fourth FET having a back gate terminal, and said switch further including a fifth FET for electrically connecting said back gate terminal to said input terminal of said fourth FET when said fourth FET is on, and a sixth FET for electrically connecting said back gate terminal of said fourth FET to said point of reference potential when said fourth FET is off.
 16. A single pole, multiple throw RF switch, comprising:at least first and second switching field effect transistors (FETs) each having gate and back gate terminals, an input port for receiving an RF signal, and an output port for providing substantially said RF signal during an ON state of the respective switching FET, said first FET being ON and OFF while said second FET is OFF and ON, respectively; and a first switch coupled to said first switching FET for coupling the back gate terminal of said first FET to the input port of said first FET during the ON state of said first FET to reduce insertion loss, and for coupling the back gate terminal of said first FET to a point of reference potential during an OFF state of said first FET to increase isolation; and a second switch coupled to said second switching FET for coupling the back gate terminal of said second FET to the input port of said second FET during the ON state of said second FET to reduce insertion loss, and for coupling the back gate terminal of said second FET to said point of reference potential during an OFF state of said first FET to increase isolation.
 17. The RF switch of claim 16 wherein said first switch comprises a third FET for coupling the back gate terminal of said first switching FET to the input port of said first switching FET when said first FET is on, and a fourth FET for coupling the back gate terminal of said first FET to said point of reference potential when said first FET is off, and said second switch comprises a fifth FET for coupling the back gate terminal of said second FET to the input port of said second FET when said second FET is on, and a sixth FET for coupling the back gate terminal of said second FET to said point of reference potential when said second FET is off.
 18. The RF switch of claim 16 wherein said point of reference potential is ground potential, and said RF switch further comprising a first shunt FET for improving isolation of said RF switch when said first switching FET is on, said first shunt FET being coupled between the output port of said first switching FET and said point of reference potential, said shunt FET being switched off when said first FET is on and being switched on when said first FET is off, and said RF switch further comprising a second shunt FET for improving isolation of said RF switch when said second switching FET is on, said second shunt FET being coupled between said output port of said second switching FET and ground and being switched on when said second switching FET is off and off when said second switching FET is on.
 19. The RF switch of claim 16 wherein each of said switching FETs comprise silicon MOSFETs operating in the depletion mode.
 20. The switch of claim 10, wherein said point of reference potential comprises ground potential.
 21. The switch of claim 10, wherein said FET comprises a depletion mode silicon MOSFET.
 22. The switch of claim 10, wherein said switching circuitry comprises a second FET for coupling the back gate terminal of said switching FET to the input port of said switching FET during said ON state and a third FET for coupling the back gate terminal of said switching FET to said point of reference potential during said OFF state.
 23. The switch of claim 10, further comprising a shunt FET coupled to the output port of said switching FET and said point of reference potential, said shunt FET being switched off during said ON state and switched on during said OFF state.
 24. The switch of claim 10, wherein said switch is adapted for use at microwave frequencies.
 25. The switch of claim 10, wherein said input port comprises a source region contact of said FET and said output port comprises a drain region contact of said FET. 